1. Field of the Invention
This invention relates to a modified polyurea-polyurethane RIM composition, molded articles of a modified polyurea-polyurethane RIM composition, and a method of forming a modified polyurea-polyurethane molded article.
2. Discussion of the Background
Polyurethane polymers prepared from an active hydrogen-containing compound and an isocyanate are widely employed in molding processes, particularly reaction injection molding (hereinafter RIM) processes. RIM articles are finding increased usage as automotive fascia.
The basic polyurethane polymer systems are typically based on an OH polyol component, an OH cross-linker and an isocyanate component. However this system suffers from long cream, demolding and cycle times, greatly increasing the processing time. Modifications to the basic polyurethane system to shorten these processing times have been achieved through substitution of the OH cross-linker with an aminic cross-linking system. Typically, thermosetting urethane polymer compositions comprise an isocyanate component with an excess of isocyanate groups and an aromatic diamine as a chain extender, to form a polyurea-type urethane polymer. Optionally, the polymer composition may also contain additional amounts of a reactive polyol to form a hybrid urea-urethane polymer. Such systems greatly decrease the cream and demolding times, therefore enabling much shorter cycling times in a RIM process.
The use of chain extenders, such as di-alkyl aromatic diamines, and more particularly di-ethyltoluene diamines and di(alkylthio)aromatic diamines are often used with isocyanate pre-polymers alone or with a polyol component to form a polyurethane/polyurea molding RIM composition (for example U.S. Pat. Nos. 4,595,742, 4,631,298, and 4,786,656). While an increase in the flexural modulus is observed through the addition of di-alkyl(thio)aromatic diamines, these compositions are still limited with respect to flexural modulus without observing "cold break" on demolding. "Cold Break" is a brittleness observed in the molded article during demolding. The presence of cold break causes the molded article to fracture on demolding. When trying to achieve a higher flexural modulus, to above 80,000 psi, by increasing the isocyanate content of the isocyanate component (i.e. higher % NCO), these materials suffer "cold break". Alternatively attempts to increase the flexural modulus by increasing the functionality of the polyol component also suffers from "cold break".
In addition to the mechanical properties of the polyurethane polymer, the processing of the polymer systems plays an important role in the usefulness of a polyurethane system. In RIM processing, a short gel time is desired to increase the productivity of the overall process. However, polyurethane systems based on an OH polyol component, an OH cross-linker and an isocyanate component had a gel time of from 5-8 seconds and a cycle time of from 3-3.5 min. Polyurethane systems using -NH.sub.2 crosslinkers and -OH polyols (for example U.S. Pat. Nos. 4,595,742, 4,631,298, and 4,786,656) reduced the gel time to about 1.2 seconds and the cycle time to 1.5-2 min. This greatly increased the productivity of RIM processes using these systems, but these -NH.sub.2 crosslinkers and -OH polyols systems suffered from an inability to increase the flexural modulus above 80,000 psi without observing "cold break", without the addition of fillers.
Polyurea systems based on amine terminated polyether resins and aminic cross-linkers have been developed (U.S. Pat. Nos. 4,396,729, 4,433,067 and 4,444,910), which afford superior heat resistance and mechanical properties. Due to the extremely high reactivity of the amine terminated polyether resin, the gel times are extremely short, in the range of 0.7 seconds and the demold times are very short. Such a rapid reaction rate makes these systems very difficult to manipulate, and also severely limits the type of RIM technique for which such a composition is suitable.
One of the major problems encountered is the RIM processes in premature gelling of the composition, causing insufficient filling of the mold, or limiting the size of the molded article due to the rapidity with which the system gels. In processing by RIM methods, it is necessary that the molding composition maintains a sufficiently low viscosity in order to completely fill the mold. After the mold is filled, the material must then polymerize very rapidly in order to reduce the demolding times. The two properties are at opposite ends since to increase the demolding times by using a rapidly polymerizing system, the mold size has been limited. By decreasing the gel time to accommodate larger molded articles, the demolding times are increased, thereby decreasing productivity of the overall operation.
One solution to increase the mold size without decreasing the rate of polymerization, is to increase the output rate of the RIM machine. In this way more material can be injected into the mold over a short period of time, which allows for the formation of larger articles. However, there is increased costs in high output RIM machines. Moreover, even this solution has its limitations and is still limited by the viscosity of the material and the rate that it can be injected by.
Another related problem associated with the longer gel times needed to accommodate larger molds, is the production of molded articles which exhibit a rubbery feel upon demold due to insufficient curing of the composition. Even though the molded article has sufficient structural integrity to be demolded, the molded article requires further curing to obtain the desired stiffness of the final product. This post-demolding curing only further adds to the processing time.
Ideally, a polyurethane system which exhibited a low viscosity during the mold filling stage, yet rapidly polymerized and cured after the mold has been filled is desired. Such a system would exhibit a non-linear increase in viscosity initially, followed by a very rapid increase in viscosity at the end.
The reactivity problem becomes more stringent in the case of Structural Reaction Injection molding (SRIM). This technology is not applicable to such systems having short reaction time, resulting in incomplete wetting of the structural reinforcement.
Consequently, research continues into systems with excellent mechanical and processing properties.